If you meet a number of founders of really big companies, in many cases they are sort of awkward, introverted people. The salesy, flashy people may be really good at convincing, but sometimes they can get away with less execution. This can be a good or bad thing depending on the type of startup and the market it is in.

There are also obvious examples of highly charismatic people who build big companies (e.g. Steve Jobs).

As an aside, I think Mike Moritz (investor in Google, Yahoo!, PayPal, etc.) also mentions something along the lines of "awkward/shy founders are often the ones who do big things" in a recent video interview (maybe the TechCrunch one?)

I am not sure what the cause is. Maybe inward focus helps build a great product and ignore feedback from people trying to influence your vision in the wrong direction? While charisma suggests being strongly attuned to others, whose feelings you may hurt by forging your own path? I dont know...